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luckyfrank
2010-Jul-17, 10:08 AM
So ive being reading 'A brief history of time' by Stephen Hawking in the chapter on black holes he states that an astronaut falling into a super massive black hole wouldnt feel much if any gravitational effect when passing the EH and wouldnt feel anything until deep inside it,

Now im confused the gravitational force at the EH is so great light is unable to escape yet our astronaut isnt being torn to shreds when he passes it and is deep within before he is spaghettified

It doesnt make sense

Tobin Dax
2010-Jul-17, 11:43 AM
Here's a "quick and dirty" response:

Spaghettification is a tidal effect. The stretching is due to the difference in the pull of gravity across your body, not the strength of the gravitational pull at the EH. Near the EH of a stellar-mass black hole, that difference is big, but it's a much smaller difference near the EH of a supermassive black hole essentially because the radius of the EH is so big. That far from the center of the black hole, the gravitational pull on your feet isn't much bigger than the gravitational pull on your head, so you don't get spaghettified.

mmaayeh
2010-Jul-17, 11:46 AM
Hi luckyfrank,

I got a feeling I should leave this question to the more advanced members but, I will give it go.

I believe the affect of spaghettification near the EH is more prevalent in a stellar black hole vs. a super massive black hole is due to gravitational gradient. If you are near a black hole that is just formed from a star that just died then the gravitational force on your toes would be much greater than the gravitational force at the top of your head so the black hole will start to rip you apart. For a super massive black hole the gravitational gradient between your toes and the top of your head is nearly zero so that you get pulled in past the EH without being spaghettified. but, when you approach the center of the singularity within the super massive black hole then, the gravitational gradient begins to increase and you get ripped apart at that point. But, this is not to say that the super massive black hole is not pulling on you weakly. Well, it is a very strong gravitational force is pulling on you and you can't escape but, along the length of your body the gravitational force is constant where as in a stellar black hole the gravitational force varies along the length of your body.

I hope this helps.

caveman1917
2010-Jul-17, 11:26 PM
Simply put, spaghettification is about the relative difference in gravity between different parts of your body, not about the absolute strength of that gravity.

luckyfrank
2010-Jul-18, 08:28 AM
Yes but still the once you pass the EH at a black hole the gravity is so strong light is unable to escape this is true of steller mass black holes and black holes at the center of a galaxy, now you'd be torn to shreds when you pass the EH of a steller mass black hole and not a black hole at the center of a galaxy it just seems strange that anyone passing the EH at a galatic one wouldnt notice much effect considering the gravity is strong enough to prevent light escaping

caveman1917
2010-Jul-18, 08:36 AM
It doesn't matter how strong gravity is. If every part of your body is pulled in the same direction [ETA: with the same strength], nothing happens to you, no matter how hard that pull is.
But when your feet are pulled harder than your head, then you will stretch. It's about the difference.

Consider holding a piece of string in your hands, one end in each hand.
Now, when you move both your hands in concert, nothing changes with the string, no matter how fast you move your hands.
It's only when one hand starts to do something else than the other that the string will stretch.

The reason it happens with stellar black holes at the horizon, is because the horizon is very close to the singularity, supermassive black holes have horizons very far from the singularity.
The horizon is irrelevant concerning spaghettification, it is only distance to the singularity (and mass of the singularity) that matters.
The fact that with a stellar black hole it happens already at the event horizon is a pure coincidence, they have nothing to do with eachother.

Strange
2010-Jul-18, 09:14 AM
Also worth noting that doesn't matter how strong the gravity is if you are in free fall; you will still be "weightless". The gravity only affects how rapidly it accelerates. In fact, this explains spaghettification:when you feet start accelerating significantly more than your head...

If the EH was solid so you could stand on it, you would be equally crushed in both cases.

Tobin Dax
2010-Jul-18, 10:56 AM
Let's toss some numbers in here. I'm going to purposefully use Newton's Law of Gravitation most of the time, but it should get the idea across well enough it's use can be forgiven.

Gravitational Force, according to Newton, is given by F=GMm/r2. Acceleration due to gravity is then a=GM/r2. G is the gravitational constant, M is the mass of the black hole, and r is the distance from the center of the black hole.
The Schwarzchild radius per unit mass is about 3 km/solar mass. Let's assume that our doomed astronaut is 1.8 meters tall (a little under 6 feet).

If the astronaut's feet are at the event horizon of a black hole with the mass of the sun (r=3 km), then his head is 1.8 meters farther away. His head is 0.06% farther out than his feet are, which means that his feet are accelerating 0.012% more than his head. That fraction of a percent might seem insignificant, but his feet are accelerating at 15 trillion m/s2.

If the astronaut's feet are at the event horizon of a 1,000,000 solar mass black hole (r=3,000,000 km), then his head is 0.00000006% farther out than his feet. His feet are accelerating 4x10-21% more than his head. That's immeasureable. His feet are still accelerating at 15 trillion m/s2, but his head is accelerating at the same rate. Outside the solar-mass black hole, his head was accelerating at a significantly slower rate than his feet, which causes the spaghettification.

The tidal stretching is due to the difference in gravitational force across the astronaut's body. That's difference is essentially nonexistant at the EH of a supermassive black hole.

luckyfrank
2010-Jul-18, 05:47 PM
Thanks for the replies thats cleared that up for me

mugaliens
2010-Jul-19, 07:45 AM
The tidal stretching is due to the difference in gravitational force across the astronaut's body. That's difference is essentially nonexistant at the EH of a supermassive black hole.

And once beyond the EH? While we cannot measure anything inside the EH from outside the EH, it's theorized our astronaut will continue to fall, and as he does, how far until he spaghettifies? Is the matter inside a BH actually infinitely compressed at a singularity, or does it exist in quark or even preon degeneracy? Regardless, there exists a point inside the EH where our astronaut will spaghettify.

Tobin Dax
2010-Jul-19, 08:13 AM
Regardless, there exists a point inside the EH where our astronaut will spaghettify.

Yes, there is. I didn't exclude that possibility. The question was about the region near the EH, as was my response.

neilzero
2010-Jul-19, 04:47 PM
As some of the others posted, we can't know what is inside a black hole from far away. Possibly we can learn some things from just outside the event horizon. Possibly the subatomic particles tend to concentrate just inside the event horizon = high density and extremely hot. My guess is medium density just inside the event horizon, with 1% (more for old black holes) of the mass within one meter of the center, and extremely hot near the center = sub-atomic particles are doing sling shot maneuvers around the center, and colliding frequently with each other.
For the super massive black hole, the accretion disk is likely much the same inside the event horizon as just outside except closer to c and more violent. I'm guessing of course. Neil

Jeff Root
2010-Jul-19, 06:04 PM
I'm guessing that your guesses are almost all contradicted by the
predictions of general relativity, which works just fine near the event
horizon and even deep inside, until you get very, *very* close to the
center. If particles could do "sling shot maneuvers around the center",
they could rise through the event horizon. Nothing can rise through the
event horizon, because of the strength of the gravitational field there.
The gravitational field keeps getting stronger as you get closer to the
center.

Nothing can orbit just outside the event horizon, because of the
strength of the gravitational field there. The gravitational field
keeps getting stronger as you get closer to the center.

-- Jeff, in Minneapolis

grant hutchison
2010-Jul-19, 08:47 PM
Possibly the subatomic particles tend to concentrate just inside the event horizon = high density and extremely hot. My guess is medium density just inside the event horizon, with 1% (more for old black holes) of the mass within one meter of the center, and extremely hot near the center = sub-atomic particles are doing sling shot maneuvers around the center, and colliding frequently with each other.
For the super massive black hole, the accretion disk is likely much the same inside the event horizon as just outside except closer to c and more violent. I'm guessing of course. NeilThis is Questions and Answers, not Questions and Guesses. So it would be useful if you didn't speculate wildly.
GR doesn't allow any trajectories that do anything you describe. Everything that crosses the event horizon is on a one-way trip to the singularity, along a time-like trajectory. Nothing hovers, nothing does sling-shots, there is no accretion disc closer than the photon sphere.

Grant Hutchison

Swift
2010-Jul-19, 09:51 PM
As some of the others posted, we can't know what is inside a black hole from far away. Possibly we can learn some things from just outside the event horizon. Possibly the subatomic particles tend to concentrate just inside the event horizon = high density and extremely hot. My guess is medium density just inside the event horizon, with 1% (more for old black holes) of the mass within one meter of the center, and extremely hot near the center = sub-atomic particles are doing sling shot maneuvers around the center, and colliding frequently with each other.
For the super massive black hole, the accretion disk is likely much the same inside the event horizon as just outside except closer to c and more violent. I'm guessing of course. Neil
neilzero,

This sort of unsupported speculation is not appropriate for the Q&A thread. Please stick to mainstream answers.

grant hutchison,

Please leave the moderation to the moderators.

grant hutchison
2010-Jul-19, 10:00 PM
grant hutchison,

Please leave the moderation to the moderators.My apologies.
Gladly. :)

Grant Hutchison

mugaliens
2010-Jul-21, 07:57 AM
Yes, there is. I didn't exclude that possibility. The question was about the region near the EH, as was my response.

You're right, of course. I was merely using your comment as a springboard for a couple of closely related and clarifying questions. :)